By Michael Birnbaum, Souad Mekhennet and Griff Witte

Brussels: An Islamic State bombmaker whose DNA connected him to November's Paris attacks was one of two suicide bombers at Brussels Airport, two intelligence officials say, the strongest link yet between two Islamic State attacks that have stunned Europe with their power and planning.

Najim Laachraoui, 24, who is believed to have prepared explosives for the November Paris attacks, blew himself up at Brussels Airport on Tuesday, according to an Arab intelligence official and a European intelligence official, speaking on condition of anonymity to discuss sensitive findings.

Laachraoui, who Belgian prosecutors on Wednesday said they still believed was on the loose, joined forces in the suicide attack with Ibrahim el-Bakraoui, 29, a Belgian with an extensive criminal record, the officials said.

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Bakraoui's younger brother, Khalid el-Bakraoui, 27, carried out a suicide bombing on the Brussels metro 73 minutes after the initial attack at the airport, prosecutors said.

Suspect Najim Laachraoui died in the Brussels airport bombing.Credit:AP

A third man who left a bomb in the airport but escaped is still at large, prosecutors said.

The missive, contained in a discarded computer, does not specifically cite recent raids across Belgium, including one that netted a key suspect in last year's Paris attacks. But its tone suggests a sense that the noose was tightening, according to Belgium's federal prosecutor, Frederic van Leeuw.

Fears that authorities were closing in may help explain the involvement of Laachraoui in the suicide bombing at the airport. Terrorism experts regard bombmakers, especially those trained in handling sensitive explosives, as among the most valuable and protected members of a terrorist organisation. It is highly unusual for them to participate in suicide attacks themselves.

Police have arrested Mohamed Abrini who admitted he was 'the man in the hat' at Brussels airport.Credit:AP

The computer message also gives apparent insight into the tactics, organisation and motivation of the militants who perpetrated the worst attacks on Belgian soil since World War II, and possibly a deeper look into the wider network linked to last year's Paris massacres.

He suggested that counter-terrorism officials had the militant on their radar long before the Paris attacks or Tuesday's bloodshed in Brussels. Interpol had also issued a "red notice", effectively an international arrest warrant, for one of the suspects at the request of Belgian authorities.

Nidhi Chaphaker, right, and an unidentified woman photographed by Georgian Public Broadcast's Ketevan Kardava immediately after the terrorist attack on Brussels Airport in Belgium.Credit:AP

He wrote that he was "in a hurry, no longer know what to do, being searched for everywhere, no longer secure", according to Mr van Leeuw's description of the message, which was not made public.

The Islamic State terrorist organisation claimed responsibility for the attacks, in which at least 34 people died and 270 were injured.

Damage to the Brussels Airport terminal in the aftermath of the blasts.Credit:Twitter

Authorities now believe that the bombers had close connections to the Paris attackers.

The same bombmaker may have been involved in both attacks, and Khalid el-Bakraoui is believed to have used an assumed name to rent a Brussels area apartment where wanted Paris attacker Salah Abdeslam's fingerprints were found last week.

People receive treatment in the debris-strewn terminal at Brussels Airport after the airport bombing.Credit:AP

The computer file does not mention Abdeslam by name, but it says the attackers feared that if they did not strike quickly, they risked winding up in prison alongside "him".

"If they drag on, they risk finishing next to him in a cell," Mr van Leeuw said, paraphrasing the content of the file.

A young girl lights a candle at the Place de la Bourse following the attacks in Brussels.Credit:Getty Images

Mr van Leeuw described the file as a "will" discovered on a computer. He did not explain why authorities believed the computer belonged to Ibrahim el-Bakraoui.

Authorities also found large stockpiles of bomb-building materials at his apartment in the Schaerbeek area of Brussels, the prosecutor said: 15 kilograms of TATP explosives, nearly 150 litres of acetone, 30 litres of hydrogen peroxide, detonators and a suitcase full of nails and screws.

Khalid el-Bakraoui was identified by his DNA in the attack on the subway, the prosecutor said.

Ibrahim el-Bakraoui was identified by fingerprints recovered at the scene of the airport blast.

Belgian media initially reported that a suspect arrested on Wednesday was Laachraoui, 24. But those reports were later retracted. His DNA was found on at least one bomb used in the Paris attacks.

The latest violence has left European leaders again scrambling for ways to plug holes in security, even though it became increasingly clear that at least one of the attackers had repeatedly passed through security nets without being detained by European authorities.

Turkey warned European authorities that one of the suspected suicide bombers was a "foreign terrorist fighter," Mr Erdogan said.

A Turkish official cited by The Associated Press later said that the suspect was Ibrahim el-Bakraoui and that in July 2015 he had been deported to the Netherlands at his request. European authorities let him go because they could not establish links to terrorism, Mr Erdogan said.

In Brussels, leaders called for new powers to fight terrorism, although it was unclear whether there would be any progress this time, since similar proposals were made, then rejected, after last year's attacks in Paris.

French Prime Minister Manuel Valls repeated calls for sweeping new powers to be given to European intelligence agencies, warning that the future of European unity is at stake.

"If the European project is running out of steam, if the populists are gaining in popularity, it's because a lot of speeches are not followed up in reality," Mr Valls said Wednesday in Brussels, criticising the vows for reform that have followed other recent terrorist attacks but yielded few concrete changes.

"In the years to come, the [EU] member states will have to invest massively in their security systems," he said.

In further signs of jitters across Belgium, sports officials called off a soccer match between Belgium and Portugal scheduled for Tuesday in Brussels "because of security concerns".

Brussels Airport will remain closed at least until Thursday, officials said. At Brussels's main synagogue, events marking the Purim holiday were called off.

Authorities had been bracing for a possible attack in Belgium for months as the country struggled to stem a tide of homegrown extremism and as Islamic State repeatedly threatened to hit Europe in its core.

The targets in Brussels - home of the European Union and NATO - also appeared to have been chosen for their symbolic value and for their ease of access.

"What we had feared has happened," Belgian Prime Minister Charles Michel said. "This is a black moment for our country."

The attackers first struck with twin bombings at the international airport, where early-morning travellers were preparing to board flights linking Brussels to cities across the continent and around the world. An hour later, a subway car transiting beneath the modernist glass-and-steel high-rises that house the EU erupted in smoke and flame.

Some of the injured lost limbs as shrapnel from the blasts radiated through packed crowds. Mobile phone video recorded in the minutes after the airport blasts showed children cowering on a bloody floor amid the maimed and the dead.

The attack at the airport could have been far worse, Mr van Leeuw said. The biggest bomb - packed inside the suitcase that was wheeled on a cart by the man now being sought by a massive dragnet - failed to go off, he told reporters.

Surveillance camera images show the man, wearing a hat pulled low, next to Ibrahim el-Bakraoui and another man - still unidentified, who is believed to have died in the blasts. All three are walking through the airport departure hall with apparent explosive-packed cases on luggage carts.

Images from a subway station revealed desperate scenes as people dressed for a day's work stumbled from the mangled wreckage into a smoke-filled tunnel.

Authorities acknowledged that they had been readying for an attack. But nothing like this, they said.

"We never could have imagined something of this scale," Interior Minister Jan Jambon told Belgian television station RTL.

In Washington, State Department deputy spokesman Mark Toner said that "approximately a dozen" Americans were injured in the blasts, but that "a number" of US citizens remained unaccounted for on Wednesday - without providing more specific figures.

The State Department also issued an alert on travelling in Europe, urging Americans to avoid crowded places and to exercise caution during religious holidays and at large festivals or events.

Europe has struggled with spillover from the churning conflict in Syria. Thousands of European citizens have travelled there to fight in a war that has become a focal point for jihadists around the world. Many have returned to Europe radicalised. Europe has vowed to confront them.

"This is a kind of scenario every capital in Europe feared since the November attacks last year. A mixture of foreign fighters coming back with experience, local sympathisers on the other hand," said Rik Coolsaet, a terrorism expert at Ghent University who has advised the Belgian government on how to fight radicalisation. "You have such a large number of soft targets, and you cannot secure all of them."